Which of the following is an example of an improper fraction?

An improper fraction is defined as one where the numerator is greater than or equal to the denominator. This type of fraction indicates a value that is either greater than one or equal to one.

In this case, the fraction 21/5 exemplifies an improper fraction because the numerator (21) exceeds the denominator (5). This means that 21/5 represents a value that is greater than one, specifically 4.2 when expressed as a decimal or in mixed number form as 4 1/5.

The other fractions listed either have numerators that are less than their denominators, such as 1/2 and 3/4, which are proper fractions, or they are equal, like 5/5, which equals one but does not fit the traditional definition of an improper fraction. Hence, among the given choices, 21/5 clearly stands out as the only improper fraction.